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From the Hi-Sky RC Club, Midland Texas

Setting Up Your Airplane

by Henry Smith

Generally speaking, “setting up” refers to a new or never-flown airplane. It is also applicable to any airplane whether you purchased it from an individual or it is one that doesn’t fly as well as it should.

There are three things to consider. The first thing is how straight the airplane is. The second is it balanced? And, last but not least, where is the center of gravity? Some of the material in this can be ignored for almost-ready-to-fly (ARF) and ready-to-fly airplanes (RTF) because you can’t control them.

Bananas, Warps, and Twist: Most, if not all, kit instructions state something to the effect that you don’t want to build a “banana.” This means simply that the fuselage must be built straight. You can do this by building and gluing the fuselage in a jig or build over the plans keeping the center line true.

I prefer to use a jig so I have the centers held on a straight line until the glue sets. If it’s an ARF, a mixture of ammonia and water soaked into the wood may allow you to correct the problem.

A better arrangement is to try for a replacement from the local hobby shop or the distributor. A ready-built airplane bought from an individual may require some cutting, patching, and reglueing.

You may have read a statement similar to this in kit-building instructions that good flight performance starts with a straight, warp-free wing. If the wing is warped but not covered, again the ammonia and water soaked into the structure may work. Simply soak the wing panel, twist opposite the warp and hold for at least 24 hours, then check for straightness.

Again, I prefer to use a jig to build wings. The jig keeps the warps almost completely out. If the wing is covered, heating the covering may pull the warp out.

Tech Editor's Note:
All of this is true, BUT, you must be careful you are not removing a deliberate warp built into the wing called Washout, that helps prevent tip stalls. Washout makes the trailing edge at the wingtips sit higher than the leading edge at the wingtips. This designed "twist" in the wing of an airplane is used to delay the stall of the wingtip until after the middle of the entire wing stalls. When the middle stalls first, the aircraft simply pitches nose down with the wings remaining essentially level. Recovery, then, is simple, providing you are at a high enough altitude to allow recovery. Always try to fly at least two mistakes high.
(This note had been adjusted for accuracy. Thank you everyone who helped bring this error to our attention.)

To check for washout, look at both tips with the wing laying flat on the building board. Editor’s note cont: With the trailing edge at the middle of the wing held down on the board, both wingtips should be slightly elevated above the board. If neither tip is elevated, then there is no washout built in.

If one tip is elevated and the other not, or even depressed, then you have a serious twist in the wing that has to be removed. Such a twist would cause a snap roll when one tip stalls before either the other or the center of the wing.

If both tips are depressed, which would mean that when you put the wing on the board, the tips would touch the board and the center trailing edge would be elevated, you have “washin,” which is very difficult, if not impossible, to fly with, so you must not attempt to fly with such a condition.

This is a good time to check the wing for balance. Place the wing on a stand that supports the wing in the center such that the dihedral points down. Add weight to the lighter side using lead, nails, or other materials until the wing balances. Be certain to glue this weight securely to the outermost part of the wing. Don’t leave this material loose unless you enjoy patching the covering. An alternate procedure is to put holes in the heavy side but be careful not to weaken the wing structure.

Alignment: The manual for the kit or ARF should cover how to assemble the stab, vertical fin, and wing to the fuselage so the model is straight. It should be very much like the diagrams. Some instructions do not have a diagram covering such as this, only a printed statement.

I think a diagram is better. You can see on the diagram what you must do. Keep in mind that all dimensions “A” must be the same, all dimensions “B” and so on. You will have to move the wing or stabilizer several times to get them the same. The important thing is that you are on the centerline of the airplane.

A seamstress tape is handy for measuring. You can drill a small hole in the metal end and anchor it on the fuselage with a “T” pin. You may have to sand the wing or stab saddle to make the dimensions “E” and “F” correct. Some kit instructions omit these measurements. But this step is important, so be sure you get it correct. Do not neglect it.

Tech Editor’s Note:
A seamstress tape is handy, but it also stretches, which means that the measurement you make may not be repeatable. The longer the tape, the greater the error. Your measuring device should be of metal or wood construction, and stiff enough to make sure it doesn’t sag or at least make sure you support it during measuring.

There is no reference to the vertical fin on either diagram. Its alignment is very important for a great-flying airplane. Be sure to use a 90-degree (a right triangle) angle to glue it in place. Also, a single fixed fin, or rudder as we normally call it, should be centered on the centerline of the fuselage.

Radio Installation: We have the airplane straight. Now let’s look at the radio installation. Generally you can follow the kit manufacturer’s directions. They may have a recommended location for the servos.

I generally try to follow the directions. They have been worked out and usually are good. Don’t mount a servo such that the pull is from the side (an exception is the throttle servo). This arrangement causes the servo to rock and flutter may result. The pull on the servo lengthwise avoids this rocking movement.

The linkage from the servo to the control surface must be straight and stiff. You don’t want any flexing here. Don’t use balsa for pushrods. It may break at a bad time causing a crash. There are better materials to use.

The servo arm and control arm should be at 90 degrees to the pushrod. The length of the servo arm and control horn should be the maximum, while still giving the required control surface left/right or up/down movement. If you do all this, you will take advantage of the servos power and not have to go beyond the normal movement of that servo.

Before we leave the radio installation, we have to set up the throttle linkage. The mechanical linkage can be one of numerous ways. I have seen a solid wire, braided wire, and a tube within a tube. Of course the wire is enclosed within a tube to prevent rubbing the fuel tank. All of these work fine for this application.

Be sure to secure the outer tube to the side of the fuselage. The throttle arm will travel approximately 90 degrees. Before the engine is mounted in the airplane, check that the throttle travel is equal fore and aft when open and closed.

Select a servo arm that has approximately the same length as the throttle arm. It may take some trial and error work to see that the throttle is closed at the lowest setting of the throttle trim and fully open at maximum throttle throw. The final adjustment will be when you start the engine the first time.

Tech Editor’s Note:
It would be best if there were no metal-to-metal contact at the throttle. The engine is the source for most of the vibration in a model aircraft and vibrating metal can cause radio waves capable of interfering with the signal from your radio and receiver. With modern radios, there isn’t likely to be a problem, but better safe than crashed.

Tank Position: The lowest position of the centerline of the fuel tank should be no more than 1/2 inch below the engine’s needle valve. The highest position of the centerline of the fuel tank should be at the needle valve. Any higher and the engine will load up at idle with a full tank and go too lean when the tank is nearly empty.

A high tank position will result in flooding and difficult starting. If the tank is too low, the engine will lean out towards the end of the flight and not hold a constant setting. The tank should be as close to the engine as possible. The fuel lines should be as short as possible. A longer line is easier to grab but remember that engine has to pull fuel from the tank with nothing but the low pressure from the venturi.

The tank should be surrounded with foam rubber to prevent foaming.

Center of Gravity (CG): We are now ready to consider the center of gravity. This is a very important step in getting your model airplane ready to go. Be sure you have the model with engine, radio, empty fuel tank, and all pushrods connected.

If you forget it or neglect this step, chances are the test flight will not be a fun time. If it is too nose heavy, the model won’t fly well. If it is too tail heavy, the maiden flight may be short.

The plans or instructions should have the CG identified as a range. This range is a distance measured from the leading edge of the wing. This will be in inches or millimeters depending upon where the kit was made. Take a small strip of covering and mark the location(s) so you won’t have to measure a number of times.

I recommend for the first few flights that you have the CG in the forward half of the range. Try to balance the airplane by moving the servos and battery. Avoid adding weight if at all possible. A high-wing airplane will be balanced upright, while a low-wing airplane will be balanced upside down.

Now is a good time to check the lateral balance of the airplane. Remember we balanced the wing before. Now we will check the entire airplane. As before, if the airplane drops a wing, add some weight to the opposite wingtip. This should be a minimal amount of weight because of our earlier work.

If you followed all that we discussed, you will have a straight and well-balanced airplane that is ready for its checkout or maiden flight. There should be few surprises on that first flight.

Set the control throws per the specifications on the plans or manual. If your transmitter has dual rates, set the low rate at 70-75% of maximum. The test pilot may prefer to make that first flight on low rates to minimize surprises.

Good luck and enjoy flying your new airplane.